It is in Virtuemart->Shop->Shop. The first tab is "Vendor" (Vendor information).
If there is not such a tab maybe you don't have a shop admin registered as a Vendor. Go to Shoppers, find the admin and choose "Is a vendor" from the options at the top. Then go to Shop again...
BTW I am new to VM and Joomla too, so maybe my answer is not the best you could have. If I am wrong someone has to correct me in order to learn from that!